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Audio usually applies to longitudinal pressure waves in air at frequencies that people can hear between 20 Hz and 20000 Hz.
This air molecule animation by Dan Russell is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Based on a work at http://www.acs.psu.edu/drussell/demos.html.
Electronic signals representing sound are also called audio.
Ultrasound at frequencies above the range of human hearing are included because microphones and loudspeakers are used and the waves travel in the same way. Insects, small animals and bats can hear these sounds. Bats and dolphins have superb echo-location abilities using sound. Blind people can learn echo-location by listening to echos from short sharp click sounds.
Ultrasound used for imaging should not be called audio although the technologies used are more or less identical.
There are many applications of ultrasound in liquids and solids.
Infrasound with frequencies below 20 Hz is fun. It's not sensed with the ear. You feel it in your chest or perhaps coming up from the floor into your seat. Movie earthquakes and explosions are made more realistic with infrasound. Enormous speakers are needed or perhaps hydraulic machines to inject vibration into the fabric of the building.
It's thought that elephants use infrasound. Their huge ears can pick it up. They make low rumbling sounds that travel much further across terrain than normal higher frequency sounds. By contrast, high frequencies and especially ultrasound do not propagate so well through air.
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